1. Field of the Invention
The present invention relates to a lens system for a video projector, and more particularly to a refractive lens system located in front of the cathode ray tube for projecting onto a screen a video image formed on a face plate of the cathode ray tube.
2. Description of the Prior Art
In designing a lens system for a video projector of a three-tube type, the correction of chromatic aberration can be generally left out of consideration, since three identical video projector lens systems are located in front of three cathode ray tubes each forming decomposed three different monochromatic images, respectively, to project them toward a screen for composing one complete color image thereon, and each projector lens system is requested to transmit only such a monochromatic image. Strictly speaking, however, the monochromatic image is practically formed by light of a plurality of different wavelengths within a narrow wavelength band, the width of the band depending on the spectral characteristic of fluorescent material of individual cathode ray tubes. Especially, the images formed by the cathode ray tubes in charge of green image and blue image, respectively, are rather inadequate to be each regarded as a monochromic image. Therefore, in a high definition video projector such as CRT for displaying characters utilized as a terminal of a computer, the chromatic aberration of the video projector lens system due to the considerable width of band of wavelengths forming the "monochromatic image" is not negligible.
On the other hand, the video projector lens system generally utilize plastic lens elements for forming aspheric surfaces thereon. A plastic lens element, however, generally shows a greater degree of variation in refractive index depending on the change in temperature, in comparison with a glass lens element. In the above mentioned high definition video projector, such a temperature dependent characteristic also causes an unnegligible problem.
Various video projector lens systems have been described in Patents or Patent Applications, such as U.S. Pat. Nos. 3,800,085 and 4,348,081, Copending U.S. patent application Ser. No. 347,032 and Japanese Patent Application Laid Open No. 56-78815.